Research On Invar Alloy Maxtrix Fe-Ni-SiO₂-B Coating By Laser Cladding

With the rapid development of the social economy,the railway transportation has gradually entered the era of high-speed,which needs high requirements for the safe operation of trains and the train braking system.In the process of frequent train braking,the brake disc bears wear and complex fatigue load,which is very easy to premature surface failure.By strengthening the surface of the traditional brake disc,not only the braking performance of the train can be greatly improved and the cost can be saved,but also the remanufacturing the failed brake disc can be realized and the service life of the workpiece can be improved.In this paper,the high wear-resistant invar alloy matrix composite coating was prepared by laser cladding technology based on the low expansion characteristics of invar alloy.The microstructure,solidification behavior,microhardness,tribology property and thermal expansion coefficient of the cladding layer were analyzed by the use of metallographic microscope,scanning electron microscope,X-ray diffractometer,TG-DSC synchronous thermal analyzer,Vickers microhardness tester,friction wear tester and static thermo mechanical analyzer.The following conclusions were obtained.(1)The main phase of invar alloy coating fabricated by laser caldding is still austenite,but the surface of the coating with many holes is uneven.The Marangoni effect in the coating can be reduced and the macro-morphology can be improved by adding surface active elements.(2)The microhardness of Fe-Ni-B alloy coating is obviously improved by the eutectic strengthening.However,when the content of B is higher than 4wt.%,the invar effect of the coating is destroyed,which makes the coating with the large thermal stress to crack.(3)The surface of Fe-Ni-SiO₂alloy coating with 1wt.%SiO₂is uneven,and has some holes.When the SiO₂content is higher than 2wt%,the surface of the coating without holes is flat and well bonded with the substrate.The addition of SiO₂with the addition of Fe₃Si/SiO₂soft magnetic particles will change the orientation of the austenite grains and at the same time contribute to the refinement of the grains of the cladding coating.With the increase of SiO₂content,the Vickers hardness of the coating can be increased about 50%.The friction coefficient of the coating keeps stable when the SiO₂content is higher than 2wt%.The average linear thermal expansion coefficient of the coating with 2wt%SiO₂content in the temperature range of 25-600?is 6.66×10^-6/?,which shows the coating maintains good invar effect.(4)The flat surface of Fe-Ni-SiO₂-B alloy coating is,well bonded with the substrate,and there is no hole in the coating.With the increase of B content,the amount of Fe₃Ni₃B precipitated in the intergranular region increases,and the ratio of the eutectic structure made up of Fe₃Ni₃B and austenite increases,the Vickers hardness of the coating increases gradually.When the B content is 4wt%,the microhardness of the cladding layer can reach 650 HV_0.1,which is 3?4 times higher than that of the substrate.When the B content is higher than 1wt%,the friction coefficients of the coating keeps stable,and the surface wear belongs to mainly fatigue wear.The average linear thermal expansion coefficient of the 2wt%B coating in the temperature range of 25-600?is 6.27×10^-6/?,which shows the coating still have good invar effect.